Rendering DEMs

Nick H

Posted 04 October 2009 - 05:42 AM

Nick H

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For a little project I'm working on I need a relief base map that I decided to try making using the new ASTER 30-metre DEMs. Rendering these has been a interesting job, the elevation range is only from about 40 metres to about 250 metres. I've used shaded relief, but of course with such a small elevation range there isn't much to shade. Shading also highlights the problem with the DEMs (see the NW corner of the map in particular).

My real problem came with choosing the colours and these are still not anywhere near right I think. I tried Color Brewer, but in the end I lifted the colours from an old shaded relief map of a part of the Alps I found on the Internet.

The rendering was done with MICRODEM and exported as a WGS84 JPG with a world file that I projected to an OSGB36 geotiff using gdalwarp.

Any comments on the colours, or anything else, would be welcomed. The terrain in this part of the world isn't dramatic, but it is important.

BioGeoMan

Posted 04 October 2009 - 11:55 AM

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With this small range of elevation, I usually stick with a gradual green to light green/white color ramp, especially in highly vegetated areas. I agree with Hans that contour intervals would be helpful.

DaveB

Posted 05 October 2009 - 09:28 AM

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When I see "Kennet" what immediately pops into my mind is "West Kennet Long Barrow".

Those colors a very strong! Especially the reds. I wouldn't use a ramp from an Alpine map for such an area. I agree with the other comments, a subtler ramp with less drastic color changes and possibly the addition of contour lines might be better. Also consider what other information is going on top of this. One resource you might look at for ideas is Imhof's Cartographic Relief Presentation.

Nick H

Posted 05 October 2009 - 11:09 AM

Nick H

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Thanks for the suggestions, all. I think I'll have to concentrate on finding a colour-ramp solution, any attempt to use terrain-shading just highlights the problems with the DEMs. Contours extracted from the DEMs reflect the problems with the DEMs and look truly nightmarish.

Finding a good colour-ramp is as you'll know and as I'm finding out not a trivial matter .

I think West Kennett long barrow might just be on the map I posted, what a part of the world that is. It's only an hour and a bit down the road and we walk there when we can, Avebury, Silbury Hill, West Kennett and Wansdyke. Oh dear, four candidates for 'Guess that Location' down the drain.

Charles Syrett

Posted 05 October 2009 - 11:32 AM

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One resource you might look at for ideas is Imhof's Cartographic Relief Presentation.

I would second this. Imhof takes a hard look at various colour ramps along with advantages, disadvantages, etc. I've attached a sample of a relief rendering I did recently that uses one of Imhof's colour ramps as a starting point. The elevation difference on this one is less than 40 metres from high to low.

You may want to keep working with the relief image as well. There are ways to deal with the artifacts and bumps, if you're willing to put in some R&D time. Study Tom Patterson's tutorials: http://www.shadedrelief.com/ His materials, along with Imhof's, give you the sum total of our planet's wisdom on relief rendering.

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Nick H

Posted 06 October 2009 - 10:03 AM

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I've done a little work on the map. The surface of the one I posted earlier was derived by merging and rendering two 30-metre ASTER DEMs, which produced very poor results. I have obtained better (I think) results by working with a subset of a processed 90-metre DEM. The new colour ramp I lifted more or less in toto from here:

but this won't stop me from following up your suggestions on further reading. The terrain-shading might be a little over the top. Now I've started to add water features, a part of the Thames and the Kennet and Avon Canal, which appear to be in about the right places. Contours I'll experiment with.

Nick H

Posted 07 October 2009 - 04:54 PM

Much better! You may be able to leave contour lines out of it in this case. Still, it might be worth giving them a try just to see if it adds to the map.

Well here goes, this is about the best that I've been able to do using contours. It certainly emphasises the topology but is a bit over the top for my taste. On the other hand, it does make clear the problems that the old canal builders faced.